In relation to a fuel cell electrically powered by electrochemical reaction between hydrogen and oxygen, various applications to portable apparatuses, automobiles etc. have been studied. The fuel cell has a structure obtained by stacking several 10 to several 100 basic constitutional units, i.e., unit cells each consisting of an electrolyte membrane, an electrode and a bipolar plate. In a process for producing a general fuel cell, the electrolyte membrane and the electrode are previously formed as an electrolyte membrane/electrode assembly (MEA), and the bipolar plate is arranged thereon. The bipolar plate is provided with passages for supplying fuel gas such as hydrogen gas, an oxidizer consisting of air or oxygen and a coolant for cooling the cell respectively on at least a single surface thereof.
While the bipolar plate must be sufficiently conductive in order to improve power generation efficiency of the fuel cell by ensuring electric connection with the adjacent MEA, sufficient mechanical strength is required in addition thereto, in order to support the laminated structure of the unit cell. Further, reduction in the thickness of the bipolar plate has also been recently required following the requirement for downsizing of the fuel cell. In addition, improvement in thickness accuracy is also required in order to reduce contact resistance between the unit cells in the laminated structure of the unit cells.
As a method for improving the electrical conductivity of a fuel cell bipolar plate, a method increasing the content of a carbon material in a molded item containing at least the carbon material and a resin binder can be listed. While the electrical conductivity can be improved by this method, the mechanical strength of the bipolar plate tends to lower if the content of the carbon material is excessively increased, and hence the ratio between the carbon material and the resin binder has generally been designed on the basis of the balance between the electrical conductivity and the mechanical strength of the bipolar plate in the actual situation.
Thus, there are proposed various techniques for improving both of the electrical conductivity and the mechanical strength of the fuel cell bipolar plate and techniques related to a fuel cell bipolar plate reduced in thickness.
Japanese Patent Laying-Open No. 63-294610 (Patent Document 1) proposes a conductive molded plate prepared by introducing thermosetting resin and carbon powder into a porous sheet of organic fiber or ceramic fiber in communication in the thickness direction of the sheet so that the electric specific resistance of the sheet is not more than 10 Ωm in the thickness direction.
Japanese Patent Laying-Open No. 2000-323150 (Patent Document 2) proposes a fuel cell bipolar plate or the like made of resin containing micro members of a carbon material for ensuring sufficient electrical conductivity by containing a larger quantity of the micro members in the outer surface as compared with the inner part and ensuring sufficient strength by forming the inner part by resin containing a small quantity of the micro members.
Japanese Patent Laying-Open No. 2001-126744 (Patent Document 3) proposes a fuel cell bipolar plate having a conductive material at least on a single surface or in the inner part of a synthetic resin molded item of graphite particles including coarse graphite particles of at least 40 to 120 μm in average particle diameter (D50%) and non-carbonaceous resin and exhibiting high electrical conductivity, mechanical strength, thermal conductivity, high dimensional accuracy and the like.
Japanese Patent Laying-Open No. 2001-52721 (Patent Document 4) proposes a fuel cell bipolar plate constituted of graphite particles and at least a single type of non-carbonaceous resin selected from thermosetting resin and thermoplastic resin with the graphite particles including coarse graphite particles of at least 40 to 120 μm in average particle diameter (D50%) and a conductor covering at least a single surface of this molded item to be excellent in gas impermeability, mechanical strength, electrical conductivity in the thickness direction and the like.
With respect to the requirement for downsizing and reduction in thickness, Japanese Patent Laying-Open No. 2005-100703 (Patent Document 5) proposes a conductive molding material, prepared by wet-forming a slurry obtained by suspending a carbonaceous base material and thermoplastic resin fiber in water and sheeting and molding the same, containing the carbonaceous base material (A) and the thermoplastic resin fiber (B) as essential ingredients with the ratio A/B (mass ratio) of 90/10 to 62/38.
Although the electrical conductivity and the mechanical strength of the fuel cell bipolar plate can be rendered compatible with each other to some extent according to each of the methods described in Patent Documents 1 to 4, these performances are not yet highly exhibited in a well-balanced manner. While a bipolar plate downsizeable and reducible in thickness can be obtained according to the method described in Patent Document 5, this method does not disclose a bipolar plate simultaneously satisfying the electrical conductivity and the mechanical strength.    Patent Document 1: Japanese Patent Laying-Open No. 63-294610    Patent Document 2: Japanese Patent Laying-Open No. 2000-323150    Patent Document 3: Japanese Patent Laying-Open No. 2001-126744    Patent Document 4: Japanese Patent Laying-Open No. 2001-52721    Patent Document 5: Japanese Patent Laying-Open No. 2005-100703